The present invention relates to a lapping article used for the grinding, fining, and polishing of glass and plastic ophthalmic lenses and, in particular, to laps made of plastic.
The lapping of glass ophthalmic lenses was originally performed by cast iron laps which were rotated and/or oscillated against a glass lens blank with loose abrasives disposed therebetween to perform grinding, fining and polishing operations on the blank. Eventually, grinding machines were developed to perform the grinding step, but the fining and polishing operations continued to be performed by cast-iron laps. Among their shortcomings, laps tended to wear rapidly, necessitating that they be frequently recut and retrued. Also, the considerable weight of the cast iron laps induced a rapid wearing of the bearings and imposed limitations on both the maximum oscillatory speed which could be attained and the energy efficiency of the lap-driving mechanism.
Eventually, the practice of using loose abrasives was phased out in favor of mounting replaceable abrasive pads on the face of the lap. Because the lap itself would then be subjected to less wear, the laps could be formed of materials exhibiting less toughness and weight than cast iron, such as aluminum and plastic for example.
Attention is directed, for example, to German OS 36 40 678 and OS-37 12 148 which describe plastic laps used in conjunction with replaceable pads. The laps described therein are of solid construction and the bottom faces thereof are provided with customary side notches and central rectangular aperture for proper mating with a face of a receiver to which the laps are to be mounted.
Due to the reduced weight of the plastic laps, the rate of bearing wear is reduced and the efficiency of the lap-driving mechanism is increased. However, room for improvement remains if further reductions in lap weight could be attained.
Furthermore, the techniques presently available for manufacturing plastic laps are limited, due to the need for achieving and maintaining a precision curvature of the pad-receiving front face of the lap. In that regard, the primary function performed by such a lap is to provide a proper shape to the abrasive pads; the pads are flexible and will assume the shape of the front face of the lap. That face must thus exhibit the true optical curvature being imparted to the lens. It is, therefore, necessary that the techniques for making the plastic laps ensure that the proper curvature of the front face will be maintained.
One way of manufacturing solid plastic laps is to extrude a solid plastic cylinder and then cut the cylinder into disks. A curved face would then be machined into one side of the disc to define a pad-supporting surface, and the earlier-mentioned side notches and central rectangular aperture would be cut into the other side of the disc. Due to its solid construction, the lap exhibits sufficient strength to maintain the shape of the abrasive pad applied to the front surface. However, laps made by that technique are very expensive. The laps cannot be made by faster and less expensive techniques, such as high-speed injection molding, because such a large piece of solid plastic would cool much too slowly in the mold and would tend, as it cools, to shrink excessively, as well as to shrink at different rates in different directions, whereby the resulting lap would be warped and the molding cycle excessively long.
Therefore, it would be desirable to not only provide a lighter weight lap but also to provide a lap which can be made by a faster, less costly, and more accurate technique. Such a semi-finished plastic lap would also desirably be easily machinable to exact front-surface curvature on standard lap-cutting machines as has hereinbefore been done with solid plastic and aluminum laps to match the exact curvature desired on the lens surface.